The invention relates generally to digital information transmission systems and specifically to techniques for controlling signal clipping over a discrete multi-tone channel such as an asymmetric digital subscriber line.
The background is described in connection with an asymmetric digital subscriber line communications system, although the principles disclosed may be applied to other multi-tone or multi-carrier systems.
Digital Subscriber Line (xe2x80x9cDSLxe2x80x9d) modems provide high-speed communication links over existing copper twisted-pair wiring connecting the telephone company central office to the residential customer. With one DSL modem in the CO and one in the home, a customer can connect to the backbone data network (e.g. the Internet) and achieve data throughput as high as 8 Mbits/second. Such a high data rate can facilitate new Internet applications, entertainment, education, etc.
Asymmetric Digital Subscriber Line (xe2x80x9cADSLxe2x80x9d) signaling was established as the ANSI T1.413 standard described in xe2x80x9cT1.413-1995: Telecommunicationsxe2x80x94Asymmetric Digital Subscriber Line (ADSL) metallic interfacexe2x80x9d, 1995, incorporated herein by reference. Discrete Multi-Tone (xe2x80x9cDMTxe2x80x9d) modulation was chosen as the transmission method for ADSL. DMT is often called the line code and is a multicarrier transmission method that partitions the available transmission bandwidth into many narrow bandwidth subchannels over which parallel data streams are transmitted. Because the different subchannels are affected differently by the transmission channel, the quality of the individual subchannels can be estimated and the data density of each subchannel can be adjusted to achieve maximum throughput.
Although the performance of a DMT system can be extraordinary, the complexity of the standard has precluded timely deployment of ADSL modems. As such, designs for simplifying modem implementation within the constraints of the ADSL standard are of significant merit.
ADSL modems apply a large amount of digital and analog signal processing to achieve data throughput that can be very near the theoretical channel capacity. Because of the Gaussian nature of DMT signals, it is necessary to support very high peak-to-average ratio (xe2x80x9cPARxe2x80x9d) values to prevent distortion and ensure high performance. However, to reduce modem complexity and cost, DMT modems are often designed to support lower PAR values. This savings in complexity often causes clipping of the DMT signal, which can reduce performance.
Clipping of the signal, or voltage waveform, occurs when large amplitude values are reduced to some maximum value that is less than the original value. This might occur when the signal contains amplitude values that exceed the maximum value supported by a transmitter circuit or algorithm, and hence are limited to the maximum allowed value.
In ADSL modems, some of the degradation caused by clipping is mitigated by forward error correction in the form of Reed-Solomon (xe2x80x9cRSxe2x80x9d) coding, or RS coding combined with interleaving. RS coding combined with interleaving provides the greatest protection against clipping but requires a large amount of memory and causes a long delay and high latency that is unacceptable for certain applications. RS coding without interleaving removes latency problems but also provides less protection against clipping. It is desirable to altogether avoid implementation of RS decoding at the receiver for certain applications to reduce complexity and cost.
The ADSL standard allows the RS encoding to be turned off by setting the number of added parity bytes to zero. If another method of mitigating clipping effects is employed, the RS decoding operation can be eliminated without sacrificing performance. The elimination of the RS decoding operation allows implementation of the ADSL standard on a wider variety of general purpose programmable platforms.
Prior methods have been proposed for solving the DMT clipping problem. They typically involve application of complicated signal processing algorithms in the transmitter. In general, a DMT transmitter might offset the impairments generated in the Analog Front End (xe2x80x9cAFExe2x80x9d) by applying techniques to prevent clips in the AFE. However, most of these techniques require special coordination of actions by the receiver that does not fit within the confines of the ADSL standard.
As such, a technique is needed that fits within the ADSL standard and can be implemented at the receiver to allow performance gains with any ADSL standard modem.
The present invention comprises a method and device for dealing with clipping problems wherein the receiver compensates for standards compliant transmitted signals using a relatively low-complexity Analog Front End (xe2x80x9cAFExe2x80x9d).
According to one embodiment, a clipping estimation and correction method is disclosed that operates within the constraints of the T1.413 ADSL standard and eliminates the dependence on Reed-Solomon decoding and interleaving for adequate performance when the transmitter clips the transmitted signal. The method involves the steps of measuring the signal to noise ratio (xe2x80x9cSNRxe2x80x9d) in all operating subchannels of the transmission channel; transmitting the SNR measurements for the operating subchannels to a transmitting device on the channel; communicating data over the operating subchannels using the SNR measurements to affect the throughput of data transmitted over the channel; exploiting the lowered throughput of the transmitted data to reliably compute an estimate of the clipping that occurred in the transmitter; applying the estimate of the clip to construct a compensation signal; and adding the compensation signal at the receiver to correct the clip that occurred at the transmitter.
The method can also include the steps of initializing the transmission subchannels by forcing fewer bits than they are capable of carrying and distinguishing DMT frames that have been clipped in the transmitter to make a threshold comparison that determines when a clip has occurred. If a clip has occurred, correction is applied. Otherwise, the received data needs no correction and is passed on to the user without modification.
Also disclosed is a method of compensating for clipping of signals transmitted over a digital subscriber line (xe2x80x9cDSLxe2x80x9d) system wherein the signals are modulated using a multi-carrier transmission signaling technique over multiple subchannels of the DSL system and the signal to noise ratio (xe2x80x9cSNRxe2x80x9d) of the subchannels vary from high to low. The method comprises the steps of transmitting a signal on the DSL system; analyzing the signal to determine if it has been clipped during transmission; and computing an error compensation signal for the signal if it is determined it has been clipped during transmission.
In another embodiment, a method of estimating the amplitude and location of a clip is disclosed. The method involves the steps of initializing the subchannels to create a compensation signal and adding the compensation signal in the receiver to enhance performance during decoding of the clipped frames.
The clipping estimation and correction methods of the present invention alleviate the need to compensate for transmitter clipping effects using forward error-correction coding. Thus, the methods of the present invention can be implemented on any modem platform, even one that does not support Reed-Solomon decoding.
According to another embodiment, a general purpose programmable modem that implements the T1.413 standard without Reed-Solomon decoding capabilities is disclosed. The modem supports T1.413 standard transmission/reception functions over a discrete multi-tone communications system and can be linked to an independent processing system such as a desktop or notebook computer. The modem comprises an interface having an analog side and a digital side with the analog side coupled to the communications system. The modem contains a signal processing device for transmitting and receiving signals to and from the communications system. The signal processing device has a system side and a user side with the system side coupled to the digital side of the interface. The interface is coupled to the user side of the signal processing device and forms a communications pathway between the independent processing system and the ADSL modem.
The modem incorporates the clipping error estimation and correction functions of the present invention. In particular, according to one embodiment, the ADSL modem contains stored program segments capable of causing the signal processing circuit to analyze original signals received from the communications system to determine if they have been clipped during transmission; compute error compensation signals for signals that are clipped during transmission; and add computed error compensation signal to the original signals received to obtain corrected analog signals.